This invention relates to electronic device fabrication processes. More specifically, the invention relates to chemical vapor deposition processes for forming dielectric layers in high aspect ratio, narrow width recessed features.
It is often necessary in semiconductor processing to fill or line high aspect ratio gaps with insulating material. This is the case for shallow trench isolation (STI) layers, inter-metal dielectric (IMD) layers, inter-layer dielectric (ILD) layers, passivation layers, etc. As device geometries shrink and thermal budgets are reduced, void-free filling or uniform lining of high aspect ratio spaces (e.g., AR>6:1) becomes increasingly difficult due to limitations of existing deposition processes.
One approach to gap fill is high-density plasma chemical vapor deposition (HDP CVD). HDP CVD is a directional (bottom-up) CVD process that is used for high aspect ratio gap-fill. The method deposits more material at the bottom of a high aspect ratio structure than on its sidewalls. It accomplishes this by directing charged dielectric precursor species downward, to the bottom of the gap while simultaneously removing deposited material from the trench top through sputtering by the use of biased RF power applied to the substrate. Nevertheless, HDP CVD gapfill results in the formation of cusps, also known as overhangs, at the entry region of the gap to be filled. This results from the non-directional deposition reactions of species in the plasma reactor and from sputtering and redeposition processes. The directional aspect of the deposition process produces some high momentum charged species that sputter away bottom fill. The sputtered material tends to redeposit on the sidewalls. As a result, the entry region of a high aspect ratio structure may close before bottom-up fill has been completed, leaving voids or weak spots within the structure. This phenomenon, known as “pinch-off,” is exacerbated in narrow features. This problem is exacerbated in small features. Limitations due to overhang formation become ever more severe as the width of the gap to be filled decreases and the aspect ratio increases.
In addition to undesirable formations inside the feature, a peak of dielectric material often called a “top hat” forms on the top surface of the substrate on either side of the features. Top hats are deposits of material in the shape of a peak that slopes downwards towards the entry to the gap. If not removed, the top hats may “shadow” subsequent deposition because they can restrict the angles of incidence with which the deposition species must approach the gap in order to achieve bottom-up fill. The shadowing effectively makes the aspect ratio of the gap even higher.
To overcome limitations associated with the formation of overhangs and top-hats, HDP CVD processes often rely on in-situ plasma etch steps to remove the undesirable material. Some of these in-situ plasma etch processes use high-energy ions (e.g., helium from the HDP plasma) to create a significantly anisotropic sputter etch. Other in-situ plasma etch processes use chemically-reactive etch gases (e.g., nitrogen trifluoride, NF3) to create a significantly isotropic plasma etch. After a layer of dielectric partially fills gaps on a substrate, the plasma species etch the layer to remove overhangs and top-hats and open the gap for further deposition. However, these etch steps may be inappropriate in some applications.